Looking at their campaigns, it seems that Kennedy was more progressive than Nixon. Kennedy for sure had loftier goals and Nixon at the time seemed to be campaigning on keeping the Eisenhower presidency alive in his own. To leave it at that would leave you without much since that's not the whole picture.

Kennedy seemingly would land himself in the moderate category of the modern Republican party, despite being a Democrat. He didn't grow his government huge or spend a ton of money (except military). From insiders he was a fiscal conservative. Even some of his actions were seen as counter to what liberals then and today would expect from a Democrat president. But not so much so that he would have been seen as a turncoat apparently.

Nixon is, and always will be, a tough classification. He ended the war in Vietnam (ostensibly a liberal move despite the war being lost by then) and seemed to favor liberal economic ideas, while talking a conservative rhetoric.

The most important thing I learned getting you this answer though is that position on the political spectrum changes in relation to where the American people are on the spectrum. If we're a generally more liberal society, and strong conservative will sit further right than if the society was more conservative.

Where I got Kennedy's stance: http://www.outsidethebeltway.com/john-f-kennedy-a-conservative-no-not-really/